“The grass and the vines and the willow trees were all so lush and vividly green that he was slightly awed by them. Their location within an alcove of a cliff made all of it more remarkable. It was such an unexpected place for something so beautiful, like an oasis in the middle of a barren desert." -  Katie Lynn Johnson

Wander into Big Morongo Canyon or another desert oases and you might imagine that you have been transported to another world. You will find yourself surrounded by lush, leafy fan palms, cottonwoods, willows, and other water-loving trees. In an oasis, the desert’s wide-open views and more sparsely vegetated landscapes give way to a thick tangle of leaves and branches. To be very clear though deserts are not barren. Far from it. Still, there is an alluring quality of desert oases. Here, the promise of cooling shade, the concentration of plant and animal life, and a jungle-like character draws us to them. Each oasis is different, each offers something new to discover.

Water in a desert is an enigma. By definition deserts are places where little rain falls, yet desert oases exist due to an extravagance of water, sometime at the surface, sometimes hidden a few meters below the surface, the extravagance revealed by large leafy trees that make no effort to conserve this otherwise precious and limiting resource. Desert plants possess a myriad of adaptations to conserve water, ranging from small leaves covered with thick waxy coats or fine hairs, reduced leaves that are drought deciduous, only emerging again when rains soak the desert soils, to having no leaves at all (cacti, smoke trees) and photosynthesizing, converting carbon dioxide to sugars and proteins, at night when temperatures are cooler (cacti – using CAM metabolism). Palm trees, willows, cottonwoods, and mesquite have none of those adaptations. They sport larger leaves flaunting their access to year-round water.

To understand where that water comes from, we need to think deep in time. During the Pleistocene Epoch there were repeated cooler, wetter ice ages followed by warmer and drier intervals. Those wetter-cooler periods were times when water seeped into the ground, oozing into the porous desert sands, gravels, and fractured bedrock, filling the underground air pockets, and so banking that excess water in aquifers. That hidden water slowly moves through cracks until it reaches an earthquake fault. Here the periodic but ongoing movement of the earth’s crust along transverse earthquake faults grinds the bedrock into a fine powder (called fault gouge), too fine to allow water to move through it. Underground water can build up behind faults which act as dams, sometimes pushing the water to the surface. When that happens, an oasis is formed. Every oasis in the Mojave and Colorado Deserts is associated with an earthquake fault. Importantly, the water feeding oases should be thought of as “fossil-water,” meaning it has an ancient source and is not currently being replenished.

There is another relictual - Pleistocene Epoch - character of oases. During the cold and wetter ice ages, species escaped the snow and ice and glaciers by heading south and down mountain slopes into our deserts. After the climate shifted back to warmer and drier conditions, many of those species retreated north and or back up into the mountains. Except some found these oases as perfectly suitable and so stayed. Evidence of the climate-refugial character of oases includes western fence lizards in 49 Palms Oasis (Joshua Tree National Park); fence lizards, Stebbins’ legless lizards, Gilbert’s skinks and Baja California tree frogs in the Big Morongo Canyon oasis; and western skinks in the Murray Canyon oasis (Palm Canyon – Coachella Valley). For each of these lizards and frogs, their closest conspecifics are found more toward the coast or higher up mountain slopes. The oases provide the cooler and wetter conditions they need, a refuge from the hotter and drier surrounding desert.

Oases in the Colorado Desert are typically dominated by desert fan palms, Washingtonia filifera, with honey mesquite, Prosopis glandulosa var. torreyana occupying the periphery of the oasis. When water is present here it typically is saturated with dissolved minerals, often salts of boron and fluoride. Okay to drink if dying of thirst is the only alternative, but otherwise to be avoided. Apparently, fluoride salts are difficult for our bodies to distinguish from calcium, so the fluoride is stored in places otherwise designated for calcium, such as teeth and bones. If consumed over some length of time your teeth will turn brown, become pitted, and then fall out, and your bones will become brittle. Salty crusts deposited on the soil beneath the palm trees are a giveaway that the water is not fit to drink. Yet the palm trees thrive; apparently their lack of teeth and bones allows them to avoid any deleterious effects! Unfortunately, with some notable exceptions — Thousand Palms Canyon and Palm/Andreas/Murray Canyons in the Coachella Valley, Borrego Palm Canyon in Anza Borrego Desert State Park, and 49 Palms — Colorado Desert oases are drying up. The aquifer water is no longer being pushed to the surface and there are few if any baby palms germinating to replace the aging adult trees. Increased aridity, reduced snowpacks, and increasing heat, along with water extraction for human uses are the likely causes.

In the Mojave Desert fan palms are mostly absent unless planted by people. Here cottonwoods, Populus fremontii, and willows (several species of Salix), are joined by honey mesquite. Where they occur, oases with cottonwoods and willows support a much more diverse insect and bird community compared to oases only populated with fan palms, especially for both northward and southward migrants. Many such migrant birds would perish without being able to “re-fuel” in these oases. The different compositions between typical Colorado Desert versus Mojave Desert oases might be explained by temperature and chemistry. Young palms may not be able to tolerate the freezing winter conditions in the Mojave Desert. Willows and cottonwoods may not be as able to tolerate the salinity of some of the Colorado Desert oases.

The Big Morongo Canyon desert oasis is among the most species-rich wetlands in California, perhaps even in all western North America. However, it is changing. Back in the 1980s when I first visited Morongo it was wet, really wet. There were sora and Virginia rails along what was then called the “marsh trail.”  New cottonwood and willow sprouts were common. Except for planted palm trees, there were no palms in the oasis proper. Today the rails are gone, new willow or cottonwood seedlings are gone or exceedingly rare, and the adult tree mortality seems to increase every year. Palm trees and palm tree seedlings are common. This drying follows a trend common to so many other desert oases. The influx of palm trees may be an indication that a cold temperature barrier has been breached by modern climate change. The Big Morongo Canyon oasis is located within the ecotone separating the Colorado and Mojave Deserts. The current warming and increased aridity trajectory may be nudging this oasis closer to the Colorado Desert in terms of what species can continue to thrive there, explaining the invasion of palm trees. The Big Morongo Canyon wetland is in a process of change, of re-calibrating to a new reality. Change can be uncomfortable for those of us who remember what it once was, but nature is not static. Rather change is part of nature. Think of what Morongo might have looked like during the last ice age. Wild camels and horses coming in for a drink. Mastodons and giant sloths foraging in the wetland. Saber-toothed cats waiting to pounce on unwary prey.  Change is part of nature, as are we.

Nullius in verba

Go outside, tip your hat to a chuckwalla (and a cactus), think like a mountain, and be safe